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JC-120 Distortion not really sounding distorted

Started by DrGonz78, October 12, 2014, 03:52:48 AM

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Now I know that Jazz Chorus amps are not known for their distortion sounds, but this one is not sounding like distortion at all. First off I have traced the signal through all the preamp circuit on channel 2. The master volume on ch2 was a bit dirty but cleaning that up got the pot working great now. Bass pot is also a tiny bit dirty too so it will need a cleaning no doubt. I intended to clean the pot first to make sure it was just a dirty pot and not a symptom. When you activate the distortion sound it is very scratchy when turning the pot, so I guess it needs a cleaning. Checked the distortion foot switch jack and it was not making great switching contact. I fixed that temporarily by cleaning and then clipping alligator clip to make sure it makes great contact. B+ = 42.3vDC B- = -42.3vDC approx. Clean sound is great on both channels and it is definitely something in the preamp that is not working well.

Anyhow I scoped a sine wave signal through the source and drains of Q6 & Q7 just fine. I can see even some clipping occur on the top parts of the signal when I dial up the distortion pot. C23 has signal on both sides with the dist. pot off and when turned on it will be reduced to a flat line as you turn it all the way up. I scoped the output and it is a nice healthy loud sine wave. Scope on the output does not show any significant clipping when I turn up the dist. pot all the way. So far all the transistors test good (Q1 Q2 Q4 Q5 Q7 Q8) and D4/D5 are good. So what sort of troubleshooting can I do at this point? I am really not sure what this could be at the moment so I am a little stuck. Any help greatly appreciated  :dbtu:.

My first action feels like I need to clean the pots more as it really helped the main volume (ch2) work correctly. So I will start with that next just to be sure. Also, the Serial # is 950166 and the schematic attached is accurate to this amp. I also remember Illaya had this exact circuit and felt he needed to repair the distortion. >>>http://www.ssguitar.com/index.php?topic=3388.msg25110#msg25110

He has an MP3 on his thread there so you can hear what the distortion sounds like. Now imagine my amp with the distortion turned up all the way sounds like 2% of the actual distortion is coming through mixed with the dominating clean sound. So it sounds like just clean with a few tiny crackles here and there, horrible crackles turning pot. So time to clean the pots but if not that, then what?
"A person who never made a mistake never tried anything new." -Albert Einstein


Well I just cleaned out bass and distortion pots. I ran it through the scope and this time with a 440hz signal. I can see the clipping a bit more, but won't be able to hear it till tomorrow as it is too late to crank it up. Will report back if it helped at all.
"A person who never made a mistake never tried anything new." -Albert Einstein


Well I just went out and jammed on it a bit. It works now! I mean it still sounds like total butt, but it works.

I think all the pots in this case on that ch2 were extremely dirty causing my problems. I was posting here as a way to understand this amps distortion circuit better and I am still curious how this distortion circuit works. The amps distortion is working but I think to complete the job I am going to clean the mid and treble pots too for good measure. There is still a bit of drop out or lack of consistency to the signal of guitar coming out, might just be those other pesky pots!

Edit: I will also clean up that main(signal) in jack too for good measure.
"A person who never made a mistake never tried anything new." -Albert Einstein


So I too have come to the conclusion that this distortion is just hideous sounding, even worse than Illyaa's (Sorry I mispelled it earlier) amp sounded. My JC60 sounds a bit better than this one. Could it be that the older designed Jazz chorus amps had a touch better sounding distortion? Either way it is as good as I am getting it for now. Sorry btw to post all this without having any real problems at all. Perhaps this thread will still be useful for someone either way one day.   
"A person who never made a mistake never tried anything new." -Albert Einstein

J M Fahey

It's hideous distortion, period.

Somebody had the dumb idea that it was better to clip highs than lows, so they added 2 sets of clipping diodes, one in series with a very small coupling cap, the other pair with a larger one.

The proper way is to set the diodes to clip anything they get, and handle the pre equalization in the gain stage driving them.

And gain before distortion should be some 50X before the diodes to be certain to have them clipping anything besides the largest peaks.

Can't believe they used such a crappy circuit, instead of adding a built in Tube Screamer or similar.


Hey DrGonz,
                 FWIW, I know RJC's hold value and are listed in the hall of fame but frankly (IMHO) one of the WORST amps I have ever had the misfortune to experience.
Would be lot cheaper to purchase a Stereo Chorus unit and a Real Dirt pedal. :-X


Yeah Juan it's as if when you hit chords the distortion is fighting to clip accurately. I compare it to the sound of a poorly mixed song where compression pumping all over the place and nothing is balanced at all. Then you turn off the distortion and viola a lovely clean amp!!

Yeah Phatt I am right there with you but the chorus does sound great on these amps. Although they are tough amps to make push hard in a live jam at certain times. This was a repair deal for someone else and I can hear it now... "Well now you fixed the distortion to get working and now I never want to use that feature anyway." After repairing the amp getting the distortion working you feel as if you still need to fix it more. lol  :trouble
"A person who never made a mistake never tried anything new." -Albert Einstein